The present invention relates generally to heat transfer tubes. In particular, the invention relates to the method of manufacturing the refrigerant surface configuration of a heat transfer tube that is suitable for use in air conditioning and refrigeration system heat exchangers in both evaporating and condensing applications.
A shell and tube type heat exchanger has a plurality of tubes contained within a shell. The tubes are usually arranged to provide multiple parallel flow paths for one of two fluids between which it is desired to exchange heat. In a flooded evaporator, the tubes are immersed in a second fluid that flows through the heat exchanger shell. Heat passes from the one fluid to the other fluid through the walls of the tube. Many air conditioning systems contain shell and tube type heat exchangers. In air conditioning applications, a fluid, commonly water, flows through the tubes and refrigerant flows through the heat exchanger shell. In an evaporator application, the refrigerant cools the fluid by heat transfer from the fluid through the walls of the tubes. The transferred heat vaporizes the refrigerant in contact with the exterior surface of the tubes. In a condenser application, refrigerant is cooled and condenses through heat transfer to the fluid through the walls of the tubes. The heat transfer capability of such a heat exchanger is largely determined by the heat transfer characteristics of the individual tubes. The external configuration of an individual tube is important in establishing its overall heat transfer characteristics.
There are a number of generally known methods of improving the efficiency of heat transfer in a heat transfer tube. One of these is to increase the heat transfer area of the tube. One of the most common methods employed to increase the heat transfer area of a heat exchanger tube is by placing fins on the outer surface of the tube. Fins can be made separately and attached to the outer surface of the tube or the wall of the tube can be worked by some process to form fins on the outer tube surface.
In a refrigerant condensing application, in addition to the increased heat transfer area, a finned tube offers improved condensing heat transfer performance over a tube having a smooth outer surface for another reason. The condensing refrigerant forms a continuous film of liquid refrigerant on the outer surface of a smooth tube. The presence of the film reduces the heat transfer rate across the tube wall. Resistance to heat transfer across the film increases with film thickness. The film thickness on the fins is generally less than on the main portion of the tube surface due to surface tension effects, thus lowering the heat transfer resistance through the fins.
In a refrigerant evaporating application, increasing the heat transfer area of the tube surface also improves the heat transfer performance of a heat transfer tube. In addition, a surface configuration that promotes nucleate boiling on the surface of the tube that is in contact with the boiling fluid improves performance. In the nucleate boiling process, heat transferred from the heated surface vaporizes liquid in contact with the surface and the vapor forms into bubbles. Heat from the surface superheats the vapor in a bubble and the bubble grows in size. When the bubble size is sufficient, surface tension is overcome and the bubble breaks free of the surface. As the bubble leaves the surface, liquid enters the volume vacated by the bubble and vapor remaining in the volume has a source of additional liquid to vaporize to form another bubble. The continual forming of bubbles at the surface, the release of the bubbles from the surface and the rewetting of the surface together with the convective effect of the vapor bubbles rising through and mixing the liquid result in an improved heat transfer rate for the heat transfer surface.
The nucleate boiling process can be enhanced by configuring the heat transfer surface so that it has nucleation sites that provide locations for the entrapment of vapor and promote the formation of vapor bubbles. Simply roughening a heat transfer surface, for example, will provide nucleation sites that can improve the heat transfer characteristics of the surface over a similar smooth surface. Nucleation sites of the re-entrant type produce stable bubble columns and good surface heat transfer characteristics. A re-entrant type nucleation site is a surface cavity in which the opening of the cavity is smaller than the subsurface volume of the cavity. An excessive influx of the surrounding liquid can flood a re-entrant type nucleation site and deactivate it. By configuring the heat transfer surface so that it has relatively larger communicating subsurface channels with relatively smaller openings to the surface, flooding of the vapor entrapment or nucleation sites can be reduced or prevented and the heat transfer performance of the surface improved.
In a falling film type evaporator, spreading of liquid film on the heat transfer surface and promotion of a thin film are important to improve the ability to transfer heat.
It is desirable from a logistics and manufacturing point of view to have a heat transfer tube with an external heat transfer surface that has good heat transfer performance in both refrigerant condensing and evaporating applications in the flooded and falling film evaporator modes so that a single tube configuration may be used in both condensers and flooded evaporators.